[libvirt-users] guest A from virbr0 can talk to guest B in virbr1 but not vice versa

[Martin Kletzander]

On Tue, Jun 20, 2017 at 10:05:19AM +0200, Martin Kletzander wrote:
>On Tue, Jun 20, 2017 at 02:26:59AM -0400, Travis S. Johnson wrote:
>>Hello,
>>
>>I came across an interesting problem in my home lab a few weeks ago as I'm
>>prepping for my RHCE exam using Michael Jang study guide. I've been at this
>>for days now, and I still can't wrap my head around how two or more virtual
>>networks in default NAT configuration are even allowed to communicate with
>>each other despite what the libvirt documentation said.
>>
>>
>>Here's the excerpt I'm referring to in the wiki link here:
>>http://wiki.libvirt.org/page/Networking#Forwarding_Incoming_Connections:
>>
>>> By default, guests that are connected via a virtual network with <forward
>>> mode='nat'/> can make any outgoing network connection they like. Incoming
>>> connections are allowed from the host, and from other guests connected to
>>> the same libvirt network, but all other incoming connections are blocked by
>>> iptables rules.
>>

I did not read this properly...

>>
>>Also here's another assertion from 'The virtual network driver' section in
>>http://libvirt.org/firewall.html:
>>
>>> type=nat
>>>
>>> Allow inbound related to an established connection. Allow outbound, but
>>> only from our expected subnet. Allow traffic between guests. Deny all other

I would expect the 'traffic between guests' to mean only guests on the
same network.  Also that lines up with what's written above.

>>> inbound. Deny all other outbound.
>>

[...]

>
>Thanks for reporting this.  It's clearly a bug in libvirt.  The rules
>are in this order:
>
>  all rules for virbr0
>  all rules for virbr1
>  all rules for virbr2
>
>But what we should do instead is:
>
>  input rules for all networks
>  local rules for all networks
>  output rules for all networks
>  reject rules for all networks
>
>The problem is that we do not know how other rules look like.  So what
>we might need to do is create chains where rules for the first network
>are, then only append network rules into those chains.
>
>Would you mind filing a bug for this issue, so we can properly track it
>and don't forget about it?  I'll have a look at it in the meantime, but
>don't promise anything since I'm not that familiar with that part of the
>codebase.
>

Actually, now that I'm thinking about it, the problem is not that we
disallow the communication from virbr1 to virbr0, but the problem is
that we allow the connection from virbr0 to everywhere.

Maybe the solution would be: for each network, insert the rules on top
of the forward chain and for each started network, explicitly reject
that one.  So that after one network starts it would look like this (I'm
writing this from memory just to illustrate the idea, not actually
looking up how stuff looks):

  ACCEPT any virbr0 RELATED,ESTABLISHED
  ACCEPT virbr0 virbr0
  ACCEPT virbr0 any src:192.168.122.0/24
  REJECT any virbr0
  REJECT virbr0 any

And after second network is started, we'd have:

  # This one is new:
  REJECT virbr1 virbr0
  # These ones would be normally at the end, IIRC:
  ACCEPT any virbr1 RELATED,ESTABLISHED
  ACCEPT virbr1 virbr1
  ACCEPT virbr1 any src:192.168.122.0/24
  REJECT any virbr1
  REJECT virbr1 any
  # These are left as they were:
  ACCEPT any virbr0 RELATED,ESTABLISHED
  ACCEPT virbr0 virbr0
  ACCEPT virbr0 any src:192.168.122.0/24
  REJECT any virbr0
  REJECT virbr0 any

Cc'ing Laine so that he can weigh in as he has way more knowledge of
this part of the code =)

>>Thanks,
>>
>>Travis Johnson
>
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